By combining radioimmunoassay (RIA) and enzyme-linked immunosorbent assay (ELISA), we have developed a very sensitive immunoassay--ultrasensitive enzymatic radioimmunoassay (USERIA). Experiments were conducted to modify RIA or ELISA so that antigens or antibodies could be measured by the USERIA procedure, and to compare the sensitivity among these immunoassays. So far USERIA has proven to be the most sensitive immunoassay. It is 100- to 1,000-fold more sensitive than RIA in the detection of rotavirus, cytomegalovirus, and cholera toxin. The procedure was modified to determine covalent binding of chemical carcinogens to DNA. The USERIA procedure which measures benzo(a)pyrene (B(a)P)-DNA adducts and acetylaminofluorene (AAF) adducts was 100-fold more sensitive than RIA. As low as 3 fmole AAF DNA adducts in 10 ng DNA can be measured by non-competitive USERIA and 2 fmole in 1 microngram DNA by competitive procedures. With a similar procedure, 4 fmole of B(a)P-DNA adducts in 10 ng DNA can be detected by non-competitive USERIA, and 10 fmole in 25 microngram DNA by competitive methods.